Why is it so big? Did you blow it up? I'd much rather see it at actual pixel size. Try to keep all the pixels the same size (the clouds look like they've been scaled up.)

In general, the pixels just look kind of sloppy. Some of the lines look like they need to be straightened up a bit. The quality of this could be improved a lot by just reading a couple of basic pixelling tutorials. (This one is a good start, and addresses some of the things I think you might not be familiar with, since you're new to pixel art.)

Other than that, though, you've got a cute style and some pretty decent color choices. I think you'd benefit a lot from trying something smaller - if you're used to working with vector graphics, you're probably used to working at whatever size you want and then resizing. With pixel art, you want to pick the size you want to work at and stick to it. If you want to work this big, I'd recommend splitting up the picture into smaller pieces so that you can focus on the pixels.

Hope you find some of this helpful! Good luck and happy pixelling~